In addition, the invention relates to a support arm for a preselector of a load selector. The support arm comprises a mounting section and has a free end formed opposite the mounting section. A fastening position for a contact bridge of the preselector is provided at the free end.
On-load tap changers (in abbreviation OLTC) are generally known and conventional in the prior art. They serve for uninterrupted switching over between different winding taps of tapped transformers.
Such on-load tap changers are divided into load selectors and load changeover switches.
In a load changeover switch with a selector, such as disclosed in, for example, German Patent Specification DE 100 55 406, the selector—consisting of a fine selector and possibly a preselector—is arranged below the load changeover switch. The selector serves for power-free selection of the respective new winding tap of the tapped transformer that is to be switched over to. The load changeover switch serves for the subsequent rapid and uninterrupted switching over from the connected winding tap to the new, preselected winding tap to be switched over to.
Load selectors such as described in, for example, German Patent Specification DE 28 33 126 similarly serve, like the load changeover switch with selector, for the purpose of switching over the taps of the regulating windings of these tapped transformers under load and thus selectively compensate for voltage changes at the user. Through dispensing with the separation of the load changeover switch from the selector, load selectors can be produced more economically.
Both kinds of on-load tap changer are actuated by a motor drive for the switching over. A drive output or drive input shaft that loads a force-storing unit is moved by the motor drive. When the force-storing unit is completely loaded, i.e. stressed, it is unlatched, abruptly releases its energy and actuates, in the space of milliseconds (ms), a switching tube that then executes a specific switching sequence during the load changeover. In that case, different switch contacts and resistance contacts are then actuated in a specific time sequence. The switching contacts serve for direct connection of the respective winding tap with the load diverter and the resistance contacts for temporary connection, i.e. bridging-over by means of one or more switching-over resistances. Advantageously, vacuum interrupters are used as switching elements for the load changeover. This is based on the fact that the use of vacuum interrupters for load changeover prevents formation of arcs in the oil and thus oil contamination of the load changeover switch oil, as described in, for example, German Patent Specifications DE 195 10 809 [U.S. Pat. No. 5,834,717] and DE 40 11 019 [5,107,200] and German published specifications DE 42 31 353 A1 and DE 10 2007 004 530 A1.
German published specification DE 29 13 271 [GB 2,049,287] describes a three-phase load selector for tapped transformers. Stationary preselector contacts are mounted on an inner wall of the oil tank of the load selector. The stationary preselector contacts cooperate with preselector contact bridges that are movable relative to the oil tank and that are mounted on an insulating material part that is disposed in the interior of the oil tank and capable of limited rotation relative to the tank.
International Application PCT/EP2010/059678, published as WO 2012/003864, relates to a preselector in a tap changer. A contact support comprises at least one movable contact that cooperates with preselector contacts mounted on a surrounding cylinder. In that case, the contact support is rotatable relative to the cylinder so that, through a rotation, the movable contact can come into electrically conductive connection with different preselector contacts. The course of the angular speed of the rotational movement is in that case varied during a switching process in order to reduce arc formation when a switching process takes place.
Contact supports, which are formed similarly to the basic concept, for preselectors are also described in Applications CN 2006101116522 and CN 200610116524. A further example is also present in CN 102623201.
An arc can arise in a preselector in switching processes when contact interruption takes place. In the case of repeated switching processes in the course of the service life of the preselector this can lead to damage of the contacts and to contamination of the oil in which the preselector together with further components of the load selector are located.
In order to avoid arc formation, on the one hand the switching-over speed can be increased, which means additional demands on the drive of the preselector. On the other hand, resistances can be temporarily interposed so as to lead to a voltage reduction and thus prevention of or at least reduction in arc formation at the contact to be separated. Such resistances need room, i.e. they demand additional constructional space. Moreover, resistances lead to additional costs. In addition, the resistances are additional components that can have susceptibilities to fault and thus increase the susceptibility of the entire arrangement to fault.